1. Field of the Invention
This invention lies in the technology of the manufacture of micro-structures, which include such devices as microelectromechanical structures and semiconductor devices. In particular, this invention addresses gas-phase etching procedures, focusing on those involving the etching of silicon by noble gas fluorides, halogen fluorides, or both, all in the gas phase.
2. Description of the Prior Art
The use of selective etchants to remove sacrificial layers or regions in a multilayer structure without removal of an adjacent layer or region is a necessary and common step in the manufacture of semiconductor devices and microelectromechanical structures (MEMS). MEMS have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio-frequency communications, and the range of possibilities for these structures continues to grow. One example of such a structure is a reflective spatial light modulator, which is a device consisting of a planar array of electrostatically deflectable mirrors, each microscopic in size. The device is used as a microdisplay system for high-resolution, large-screen projection. The sacrificial layer in such a device is the layer over which the mirror material is deposited. Once the mirror structure is formed, the sacrificial layer is removed to leave gaps below the mirrors and a microhinge along one edge of each mirror to join the mirror to the remainder of the structure. The gap and the microhinge provide the mirror with the freedom of movement needed for its deflection. Devices of this type are described in U.S. Pat. No. 5,835,256 (issued Nov. 10, 1998, to Andrew Huibers, assignor to Reflectivity, Inc.). The contents of this patent are incorporated herein by reference.
The success of a manufacturing procedure for structures involving sacrificial layers depends on the selectivity of the etching process. The thicknesses and lateral dimensions of the layers, and in the case of the deflectable mirror structures the width of the gap and the integrity of the microhinges, are all critical to achieving uniform microstructure properties and a high yield of defect-free product. A factor in meeting this criticality is the quality of the etch. Performance, uniformity and yield can all be improved with increases in the etch selectivity of the sacrificial layer relative to the adjacent functional layers. Selectivity is important in both isotropic and anisotropic etching procedures. Isotropic etching is of particular interest, however, in structures where the sacrificial layer is an intervening layer between functional layers or between a functional layer and a substrate, since the bulk of the sacrificial layer in these structures is accessible to the etchant only through vias in the functional layer and etchant must proceed laterally outward from the vias. The structures described in U.S. Pat. No. 5,835,256 above require isotropic etchant for this reason. The "vias" in these structures are the narrow gaps between the facing edges of adjacent mirror elements or between a mirror edge and an adjacent feature.
Among the etchants that are used for the removal of sacrificial layers or regions in both isotropic and anisotropic etching procedures are noble gas fluorides and halogen fluorides. These materials, used in the gas phase, selectively etch silicon relative to other materials such as silicon-containing compounds, metallic elements, and compounds of metallic elements. The selectivity is not infinite, however, and can vary widely with the equipment, process, materials and reaction conditions. Xenon difluoride, for example, has demonstrated selectivities as high as 400:1 to 500:1, but any improvement in this ratio would significantly benefit the cost and reliability of the products manufactured.